Exemplary embodiments of the invention relate to an exhaust gas system of an internal combustion engine, which for routing exhaust gas has a first cylindrical exhaust gas line pipe section and a second cylindrical exhaust gas line pipe section, whereby exhaust gas enriched with the reducing agent is transferable from the first exhaust gas line pipe section into the second exhaust gas line pipe section. The invention further relates to a method for preparing a reducing agent that is introduced into an exhaust gas system of an internal combustion engine.
For exhaust gas aftertreatment, reducing agents that are liquid in the starting state, such as mineral oil fuel or aqueous urea solution, are often introduced into the exhaust gas of internal combustion engines. The preparation of the reducing agent introduced in the liquid state is problematic with regard to a uniform distribution to be achieved, and evaporation. With aqueous urea solution, there is the additional problem of releasing the ammonia, which is necessary for the selective catalytic reduction of nitrogen oxides, from the urea by hydrolysis and/or thermolysis. To solve this problem, a number of exhaust gas system variants having preparation sections, mixers, evaporators, and hydrolysis catalytic converters have been proposed. Despite the numerous proposed solutions, there is still a need for improvement with regard to the preparation of reducing agents in the liquid state, and which are introduced into the exhaust gas.
Exemplary embodiments of the invention, therefore, provide a device and a method by means of which the best possible preparation of reducing agent that is introduced into exhaust gas of an internal combustion engine is made possible.
The exhaust gas system according to the invention has a first cylindrical exhaust gas line pipe section and a second cylindrical exhaust gas line pipe section for routing exhaust gas. An injector unit for introducing a reducing agent into exhaust gas flowing through the first exhaust gas line pipe section is situated at the first exhaust gas line pipe section. The second exhaust gas line pipe section has a cylinder lateral surface, a closed first end, and an open second end, as well as an opening in the cylinder lateral surface adjacent to the closed end. An open end of the first exhaust gas line pipe section is connected in a positive-fit manner to the second exhaust gas line pipe section in such a way that exhaust gas flowing from the open end of the first exhaust gas line pipe section may flow through the opening in the cylinder lateral surface of the second exhaust gas line pipe section, in an at least approximately vertical direction with respect to a longitudinal extent of the second exhaust gas line pipe section, into the second exhaust gas line pipe section. Thus, the connecting point of the first exhaust gas line pipe section to the second exhaust gas line pipe section is directly followed by a deflection of the main flow direction of the exhaust gas enriched with the reducing agent by at least approximately 90 degrees, resulting in turbulence of the exhaust gas and good intermixing of the reducing agent in the exhaust gas. The opening in the cylinder lateral surface of the second exhaust gas line pipe section has a larger extension in the direction of the longitudinal extent of the second exhaust gas line pipe section than transversely with respect to the longitudinal extent. The opening preferably has an oval or ellipsoidal shape. However, an approximately rectangular cutout may also be provided in the cylinder lateral surface. The longitudinal extension is preferably approximately 1.5 to 4 times greater than the transverse extension. The design of a particularly stable rotating exhaust gas vortex in the second exhaust gas line pipe section is made possible due to this embodiment. This in turn allows a short design of the second exhaust gas line pipe section and a short line length to the subsequent exhaust emission control unit, and thus a compact construction of the exhaust gas system having a short reducing agent preparation section. Due to the turbulence and the resulting intermixing, a separate mixer for distributing reducing agent introduced into the exhaust gas may advantageously be dispensed with in the reducing agent preparation section, and resulting pressure loss may advantageously be avoided. The injector unit is preferably designed in such a way that it is able to spray or inject the reducing agent, which is preferably present as a liquid, into the exhaust gas in finely atomized form.
The edge contour of the open end of the first exhaust gas line pipe section preferably corresponds to the edge contour of the opening in the cylinder lateral surface of the second exhaust gas line pipe section, and the positive-fit connection of the two exhaust gas line pipe sections, which is established by welding, for example, is provided along this contour. The first exhaust gas line pipe section preferably ends on the cylinder lateral surface of the second exhaust gas line pipe section, and therefore does not protrude into the interior of the second exhaust gas line pipe section. The opening in the cylinder lateral surface of the second exhaust gas line pipe section is directly adjacent to, or situated at a short distance of preferably a few millimeters from, the end-side closure of the second exhaust gas line pipe section, thus avoiding flow-related dead zones. For closing the second exhaust gas line pipe section at its first end, a lid having a flat design and which covers the line cross-section may be provided, the normal direction of the lid being the same as the direction of the cylinder axis of the second exhaust gas line pipe section. The cross-sectional areas of the first exhaust gas line pipe section and of the second exhaust gas line pipe section are at least approximately equal, at least in the region of their connecting point, but differ from one another preferably by less than a factor of 1.5. Exhaust gas flowing from the open end of the second exhaust gas line pipe section is preferably supplied to a catalytic exhaust emission control unit such as an SCR catalytic converter, a hydrolysis catalytic converter, an oxidation catalytic converter, a nitrogen oxides storage catalytic converter, or a particle filter.
In one embodiment of the invention, the injector unit is situated at a short distance upstream from the connecting point of the first exhaust gas line pipe section to the second exhaust gas line pipe section. The distance is preferably less than one diameter of one of the exhaust gas line pipe sections, in particular less than one-half or one-third diameter. The construction volume of the reducing agent preparation section or the line lengths that conduct the reducing agent and that are used for the reducing agent preparation may thus be kept particularly small.
In another embodiment of the invention, at least one plate-shaped baffle element is situated at a short distance upstream from the connecting point of the first exhaust gas line pipe section to the second exhaust gas line pipe section within the first exhaust gas line pipe section, and reducing agent introduced into the first exhaust gas line pipe section by means of the injector unit strikes the baffle element. The plate-shaped baffle element acts as an evaporation element. The heat energy necessary for evaporating the liquid reducing agent, preferably present in the form of small droplets, is preferably absorbed by heat exchange with the counter-flowing exhaust gas. However, separate heating of the baffle element may also be provided. The impact of the droplets of reducing agent against the heated baffle element results in at least partial evaporation of the reducing agent, in addition to further diminution of the droplet size of rebounding droplets of reducing agent. As a result, distribution of the reducing agent in the exhaust gas is improved compared to a reducing agent that is present predominantly in droplet form. In addition, the evaporation process is improved due to the heat transfer from the evaporator plate to the rebounded reducing agent, which is enhanced compared to heat transfer solely from the exhaust gas. Further shortening of the reducing agent preparation section is made possible due to the at least partial evaporation of the reducing agent which already takes place at the baffle element. The baffle element is preferably situated directly above or in front of the opening in the cylinder lateral surface of the second exhaust gas line pipe section. The end of the baffle element facing the second exhaust gas line pipe section may be situated, for example, in the opening area, or offset thereto by several millimeters, at best a few centimeters, in the direction of the first exhaust gas line pipe section. The baffle element may be designed as a metal sheet that is flat or provided with surface-enlarging embossing. The metal sheet may also have a slit or perforated design, and may be provided with a coating which promotes urea hydrolysis or reduces formation of deposits, or may be roughened.
In a preferred further embodiment, the baffle element has an at least approximately flat design, a normal vector of the baffle element plane being oriented at least approximately perpendicularly with respect to the exhaust gas flow direction. Thus, a directional vector of the exhaust gas flow direction is situated approximately in the plane of the baffle element, thereby making a low flow resistance possible. The direction of the normal vector is preferably oriented at least approximately parallel to the direction of the cylinder axis of the second exhaust gas line pipe section in the area of the opening in its cylinder lateral surface.
In another embodiment of the invention, the evaporation rate of reducing agent introduced into the exhaust gas by the injector unit may be further increased by providing a plurality of baffle elements. The baffle elements are preferably oriented parallel to one another, in particular in alignment one behind the other, and situated above the opening in the cylinder lateral surface of the second exhaust gas line pipe section. Two to eight baffle elements may be provided, depending on the size of the opening in the cylinder lateral surface of the second exhaust gas line pipe section. Four baffle elements are preferably provided. With regard to the atomizing cone of the reducing agent that is delivered by the injector unit, the baffle elements are situated in such a way that each is at least partially surrounded by the atomizing cone, and therefore may contribute to the evaporation of the reducing agent. For this purpose, it is preferred for the surface area extension of the baffle elements to be selected to be different. In this regard, it is particularly preferred when the surface area of the baffle elements increases with increasing distance from the injector unit. Viewed in the direction of the surface normals, in this embodiment a baffle element covers only a portion of the surface area of the adjacent, subsequent baffle element.
In another embodiment of the invention, a cylinder axis of the first exhaust gas line pipe section is oriented toward a central region of a cross-sectional area of the second exhaust gas line pipe section in the region of the connecting point to the second exhaust gas line pipe section. The supply of the exhaust gas, enriched with the reducing agent, into the second exhaust gas line pipe section thus takes place centrally, so to speak, into the second exhaust gas line pipe section. As a result of this embodiment, two counter-rotating exhaust gas flow vortices are formed within the second exhaust gas line pipe section. This allows particularly good intermixing of the reducing agent in the exhaust gas. In addition, the actual exhaust gas flow path is enlarged with respect to the geometric length of the second exhaust gas line pipe section. Evaporation of droplets of reducing agent remaining in the exhaust gas is thus improved. In the case of urea as reducing agent, the hydrolysis or thermolysis of the urea is also improved.
In another embodiment of the invention, the distribution of the reducing agent in the exhaust gas may be further improved when the second exhaust gas line pipe section has an oval cross-sectional shape in the area of the opening in the cylinder lateral surface. In this case, two counter-rotating exhaust gas flow vortices form which in each case have an approximately circular shape in a top view. The exhaust gas flow vortices are particularly stable as a result.
In another embodiment of the invention, a cylinder axis of the first exhaust gas line pipe section is oriented toward an eccentric region of a cross-sectional area of the second exhaust gas line pipe section in the region of the connecting point to the second exhaust gas line pipe section. The exhaust gas is thus introduced into the second exhaust gas line pipe section eccentrically, in particular approximately tangentially, with respect to same. The design of an exhaust gas flow vortex that practically surrounds the entire cross-section is made possible in conjunction with a circular cross-sectional shape of the second exhaust gas line pipe section, which is preferably provided for this purpose, in the area of the opening.
The method according to the invention for preparing a reducing agent introduced into a first exhaust gas line pipe section of an exhaust gas system of an internal combustion engine provides that the introduced reducing agent is at least partially evaporated at an evaporator unit situated in the first exhaust gas line pipe section, and together with the exhaust gas of the internal combustion engine subsequently flows through an opening in a cylinder lateral surface of a second exhaust gas line pipe section into the second exhaust gas line pipe section in a direction that is oriented at least approximately perpendicularly with respect to an axial direction of the second exhaust gas line pipe section, a rotating exhaust gas flow being formed within the second exhaust gas line pipe section. Due to the at least partial evaporation made possible by the evaporator unit, particularly good preparation of the reducing agent may be achieved in conjunction with the subsequent turbulence after supplying the exhaust gas into the second exhaust gas line pipe section. The line length of the reducing agent preparation section may thus be kept short, and a compact construction is made possible.
In one embodiment of the method, a plurality of evaporator plates is provided as the evaporator unit, and the reducing agent is injected into the first exhaust gas line pipe section in such a way that it strikes all the evaporator plates. The evaporation of the reducing agent is thus improved, and a particularly high evaporation rate is achieved when the reducing agent is injected into the first exhaust gas line pipe section at a maximum acute angle with respect to a normal direction of the evaporator plates. The reducing agent is thus injected in such a way that it strikes the evaporator plates at least approximately perpendicularly, thus achieving good wetting of the evaporator plate surfaces. The angle of impact of injected reducing agent on an evaporator plate is preferably less than 45 degrees with respect to the normal direction of the evaporator plate.
In another embodiment of the method, in addition it is preferred for the reducing agent to be injected into the first exhaust gas line pipe section at least approximately perpendicularly with respect to an exhaust gas flow.
In another embodiment of the method, the exhaust gas flowing into the second exhaust gas line pipe section forms two counter-rotating vortices, viewed in an axial direction of the second exhaust gas line pipe section. Particularly effective and uniform intermixing of the reducing agent in the exhaust gas is thus made possible. The aim is to achieve a stable vortex formation that is preferably maintained, at least in part, until reaching the inlet side of an emission exhaust control component downstream from the turbulence mixing section or the housing inlet funnel of the emission exhaust control component which may be provided. This may be achieved in particular by providing an oval cross-sectional shape of the second exhaust gas line pipe section for forming two counter-rotating vortices having an at least approximately circular shape.